The invention relates to a method for the continuous production of a coarse-grained ammonium sulphate product by crystallization and to an installation for carrying out this method.
Ammonium sulphate ((NH4)2SO4) is a product which is produced on a large scale and is used mainly as fertilizer in agriculture to provide nitrogen and sulphur. In industrial terms, ammonium sulphate is produced as a by-product in some chemical processes, in particular in the production of caprolactam. In order to satisfy the requirements as a fertilizer, the ammonium sulphate should be available as a coarse-grained product (grain size d′ (RRSB) in the range of 2 to 4 mm). This ensures effective spreading power and, when being mixed with other fertilizing substances, it suppresses the tendency towards demixing.
Course-grained crystallizates can be produced from solutions in crystallizers of the DTB (Draft Tube Baffled) type or of the Oslo type. Both cases encounter the problem that the average grain size of the product crystallizate produced in the crystallizer is subjected to periodic fluctuations, i.e., phases with a high coarse grain proportion alternate with phases in which predominantly fine-grained crystallizate (e.g., grain size of less than 1.5 mm) is produced. This is the consequence of strong, spontaneous primary nucleation at elevated oversaturation in the crystallizer. In operating phases with pronounced coarse grain formation, all of the nuclei formed are dissolved by the strong fine crystal dissolution as a result of the temperature elevation in the heat exchanger of the external solution circuit of a DTB crystallizer or in the circulation of an Oslo crystallizer.
EP 0632738 B1 discloses a continuous crystallization method, in which a coarse-grained ammonium sulphate crystallizate can be produced from an oversaturated aqueous ammonium sulphate solution in a DTB crystallizer. In this method, a suspension of oversaturated ammonium sulphate solution and already formed crystallizate is constantly circulated within a DTB crystallizer in an internal circuit. By evaporation of the solvent (water), new oversaturation is continuously produced which is then broken down as a consequence of the crystallization which occurs. The vapour produced during evaporation is drawn off at the head of the DTB crystallizer. From a part which is separated from the internal circuit of the suspension by means of flow guide walls in the upper region of the crystallization chamber of the DTB crystallizer and in which in contrast to the base region of the crystallizer there is provided a clarified solution having a solids proportion consisting substantially of crystallization nuclei and fine crystals. A partial flow of clarified solution is drawn off and, after dissolution of the solids proportion contained therein, is then guided back into the base region of the crystallization chamber. In order to dissolve the solids proportion, a heat exchanger is connected into the external circuit and elevates the temperature of the clarified solution and thus the power of the solvent to dissolve ammonium sulphate. Moreover, the supply line, through which new concentrated ammonium sulphate solution can be fed into the crystallizer, also issues into the external circuit upstream of the heat exchanger. A partial flow of suspension with the proportion of solids contained therein at the desired grain size of the product crystallizate is continuously drawn off from the base region. The product crystallizate is separated from the mother solution in a thickener and by subsequent centrifugation and the mother solution is then guided back into the DTB crystallizer. In order to increase production of a sufficiently coarse-grained crystallizate and to improve production with regard to cyclical fluctuations of the grain size, in this method a crystallizate suspension of ammonium sulphate is fed at a constant inflow rate from an external source into the crystallizer, in addition to the supply of saturated ammonium sulphate solution.
EP 0632738 B1 does not disclose the manner in which the suspension is produced, i.e., whether it has been produced for instance by dissolving comminuted product crystallizate or in a separate crystallizer. It is merely specified that this suspension must satisfy specific conditions: the temperature of the suspension fed in must not exceed the operating temperature in the crystallizer. Moreover, the suspension must contain 6-24 vol. % crystallizate, wherein at least 35% of the crystals are larger than 1.2 mm, and the feed of the suspension is dimensioned such that the weight of the crystals in the fed-in suspension is in the range of 4-25% of the weight of the crystals in the suspension including the product crystallizate, which suspension is drawn off from the base region of the crystallizer. This controlled feed of crystal suspension into a crystallizer to influence the grain size is also defined as seeding.
WO 00/56416 discloses a method for controlling the grain size in continuous mass crystallization which is also provided for producing the coarse-grained ammonium sulphate crystallizate in an Oslo crystallizer or DTB crystallizer and in which, in a similar manner to the method in accordance with EP 0632738 B1, seeding is effected with an externally supplied crystal suspension. The seeding product is a crystallizate which is produced in its parameters independently of the current crystallization process and which has an average grain diameter of 0.1-1.0 mm. In this method, the temperature of the seeding product during the addition also must not be higher than the operating temperature in the crystallizer but must be up to 40° C., preferably 10-30°, less. All other feeds and re-circulations are free of solids. In particular, this means that the feedstock for producing the ammonium sulphate product crystallizate is supplied as a preheated, solid-free ammonium sulphate feed solution, and that the external circuit consisting of the drawn-off solution with a fine solids proportion leads from the crystallizer initially to a heat exchanger which by elevating the temperature of the solution serves to re-dissolve the solids proportion, before the solid-free solution is then guided back into the crystallizer. The heat energy for heating the heat exchanger is provided by means of the vapour which is drawn off from the crystallizer and is brought to a higher temperature level initially by means of vapour compression. From the base region of the crystallizer, a suspension with a solids proportion at the desired grain size is drawn off in a continuous manner and is separated by centrifugation into product crystallizate and mother solution, wherein the mother solution is conveyed into an intermediate vessel from where it is fed back into the circulating line of the external circuit of the crystallizer. The seeding product is preferably added in a quantity, the solids proportion of which amounts to 5-30 wt. % of the solid discharged from the crystallizer in each case. The solids proportion of the seeding product can be produced e.g., by mechanical comminution of a portion of the product crystallizate and/or by a separate crystallization stage.
JP 2005-194153 A discloses an installation, designed as a DTB crystallizer, for the production of ammonium sulphate crystallizate, in which an external circuit for clarified solution is provided which is connected to a clarifying zone of the crystallizer and into which optionally there is incorporated a heat exchanger or a supply line for solvents (e.g., water or undersaturated ammonium sulphate solution) for the purpose of dissolving the contained solids proportion. Moreover, the clarifying zone is connected directly or indirectly via the external circuit to a further discharge line, by means of which a suspension containing only fine crystallizate as the solid can be discharged completely from the process, in order to obtain, e.g., fine-grained ammonium sulphate product. Furthermore, connected to the uppermost part of the clarifying zone is also a third discharge line, by means of which any excess crystallization nuclei and superfine crystallizate can be drawn off and guided into a collecting vessel where required. In order to dissolve the solids, solvents are added to the collecting vessel from where the solution obtained is guided into a neutralisation vessel, in which it is added to sulphuric acid and ammonia and is heated by the neutralisation reaction associated therewith. The heated solution is then fed into the crystallizer.